1. Field of the Invention
The present invention relates to control of a hybrid vehicle, and particularly to control of a vehicle generating a creep torque.
2. Description of the Background Art
In a vehicle having a multi-stage or non-stage transmission, a creep torque that propels a vehicle at an extremely low speed occurs even when an accelerator pedal is not pressed down, as long as a drive position (forward or reverse gear) is selected. An electric vehicle that generates a vehicle drive power by an electric motor employs a structure in which the drive power generated by the motor generates a creep force. A hybrid vehicle that generates a vehicle drive power by both an engine and an electric motor employs a structure in which the drive power generated by at least one of the engine and the motor generates a creep force.
Japanese Patent Laying-Open No. 2006-050811 has disclosed an electric vehicle that can appropriately perform creep-cut according to a situation of a vehicle when a brake pedal is pressed down so that a vehicle drivability can be improved and an electric power consumption can be reduced.
The electric vehicle disclosed in Japanese Patent Laying-Open No. 2006-050811 is an electric vehicle that generates a creep torque from the drive power generated by the electric motor when a predetermined condition is satisfied. In this electric vehicle, a creep torque command value is primarily set according to a vehicle speed during generation of the creep torque. When the brake pedal is pressed down, a torque upper-limit value is set according to a press-down degree of the brake. A smaller one of the primarily set creep torque command value and the torque upper-limit value is set as a final creep torque command value. The setting of the torque upper-limit value is changed according to the vehicle situations such as changes in vehicle speed and press-down amount of the brake. Particularly, when it is estimated from the vehicle situations that a driver intends to reduce a speed, the final creep torque command value is set smaller than the primarily set creep torque command value.
The electric vehicle disclosed by Japanese Patent Laying-Open No. 2006-050811 includes a brake sensing unit for sensing a brake pedal operation by the driver, a vehicle speed sensing unit for sensing a vehicle speed of an electric vehicle, a first creep torque setting unit for setting the creep torque command value according to the vehicle speed based on a predetermined basic characteristic line in an operation of generating the creep torque, and a creep torque cut unit for setting an upper value of the creep torque according to the press-down amount of the brake when the driver steps down the brake pedal. The creep torque cut unit includes a plurality of creep torque upper-limit setting units for setting the upper limits of the creep torques based on a plurality of predetermined upper-limit value setting characteristic lines that set the upper limit values of the creep torque according to the press-down amount of the brake, respectively, a creep torque upper-limit value setting selection unit for selecting one from among the plurality of creep torque upper-limit value setting units according to the vehicle situation of the electric vehicle, and a second creep torque setting unit for setting the torque upper-limit value as the command value of the creep torque when the creep torque command value set by the first creep torque setting unit is larger than the torque upper-limit value set by the selected creep torque upper-limit value setting unit.
In the electric vehicle disclosed in Japanese Patent Laying-Open No. 2006-050811, when the brake pedal is pressed down during generation of the creep torque, the creep torque can be cut owing to provision of the upper limit value of the creep torque, and also the upper limit value can be set by selectively using the plurality of the characteristic lines for setting the upper limit values of the creep torque according to the vehicle situation. Therefore, the creep torque can be appropriately generated according to the vehicle situation even when the brake pedal is pressed down so that the drivability can be improved, and the wasteful power consumption can be reduced.
In the hybrid vehicle that uses, as the drive power source(s), at least one of the engine and the electric motor coupled together via a power splitting mechanism having a planetary gear mechanism, when the creep torque can be generated from only the output of the engine, it is not necessary to generate the torque from the motor. When the creep torque is reduced during stop of the vehicle in the above state, a pressing force between gears of the power splitting mechanism lowers so that a gear rattle may occur.
However, Japanese Patent Laying-Open No. 2006-050811 has neither disclosed nor suggested a problem of such a gear rattle as well as measures for it.